Update docs to use working image
Update docs to use working image

The image rohan/memcached-tiny no longer exists. Using nginx instead.
Also replacing spotify/cassandra with nginx since that takes too long to pull.

molindo/helios,dflemstr/helios,kidaa/helios,stewnorriss/helios,stewnorriss/helios,DagensNyheter/helios,mattnworb/helios,DagensNyheter/helios,stewnorriss/helios,kidaa/helios,molindo/helios,dflemstr/helios,dflemstr/helios,udomsak/helios,spotify/helios,udomsak/helios,spotify/helios,skinzer/helios,gtonic/helios,spotify/helios,udomsak/helios,mattnworb/helios,DagensNyheter/helios,skinzer/helios,gtonic/helios,mavenraven/helios,mavenraven/helios,skinzer/helios,gtonic/helios,mavenraven/helios,kidaa/helios,mattnworb/helios,molindo/helios